This invention relates generally to unloading systems for containers that are used to store or transport particulate materials. More particularly, the invention relates to a method and apparatus for cooling the entraining gas used in a pneumatic transport system for unloading particulate materials from such a container.
Many particulate materials are conveniently transported by truck, although they may also be transported by railcar, barge or by other means. Trucks that are used to transport such materials may include a tractor and an attached trailer having a tank or other container mounted thereon. Similar containers may be used to store particulate materials. Frequently, these containers are referred to as pneumatic containers because of the pneumatic method, involving gas- or air-entrainment, by which they may be loaded and unloaded. Materials that are generally stored or transported in pneumatic containers include agricultural products such as grain, corn kernels, beans, flour, sugar, peanuts and the like, and intermediate products for various industrial uses such as plastic pellets, coke, lime, silica gel, powdered acid resins, rare earth powders and powdered alumina, and many others.
Pneumatic containers generally include one or more product compartments that are usually cylindrical or spherical in shape in order to facilitate unloading by a method which involves pressurizing the compartments. Cylindrical or spherical product compartments are also generally easy to completely empty. Each product compartment is provided with a discharge hopper that may be generally cylindrical or conical in shape. The product compartments are generally enclosed by a sheet metal sheath, especially when the container is mounted on a trailer or other transport device, which sheath provides an aerodynamically efficient outer container surface.
One type of known construction of such pneumatic containers comprises one or more generally cylindrical and horizontally disposed product compartments which are arranged along a common horizontal axis in at least partial fluid communication with each other. A cylindrical or conical discharge hopper is provided for each product compartment, and the axis of each such discharge hopper intersects the product compartment with which it is associated generally at right angles to the axis of the cylinder of the product compartment. Each discharge hopper has a material outlet at the bottom and a valve which controls the entry of material into the outlet. An unloading system is also provided which includes a blower or other mechanism for pressurizing air or another gas. The blower provides the energy required for unloading the material from the container in the form of compressed air or another gas. One end of a pressurized gas conduit is attached to the blower and the other end to a pressurized gas inlet in the container. Operation of the blower will compress air or gas and move it through the pressurized gas inlet into the container, thereby increasing the pressure of the air or gas above the material in the product compartment or compartments in the container in order to assist in discharging material through the hopper outlets. One end of a material conveying conduit is also attached to the blower and extends past and connects to each material outlet so that when the product compartment has been pressurized, air or another gas may be directed into the material conveying conduit to entrain material passing through each material outlet and carry it to the discharge end of the conduit. The container may be mounted on a frame which is supported by the chassis of a trailer.
Although the conventional pneumatic container operates satisfactorily to discharge many types of particulate materials, problems of product degradation may arise if the materials are temperature sensitive. Because an increase in pressure of a gas is directly proportional to an increase in its temperature (so long as its volume remains constant), pressurizing the air in the container will increase its temperature, and this increased temperature will be transmitted to the particulate material. Generally, the air that is compressed and blown into the container to assist in the unloading of particulate material will be heated by about 200xc2x0 F. by the compression process. If the material is temperature sensitive, such as plastic pellets, for example, pressurizing the air in the container may even raise the temperature of the material past its melting point.
Plastics are generally transported in pellet or powdered form. When the pellets are subjected to high temperatures in the pneumatic unloading process, the plastic softens and tends to coat the walls of the discharge piping with long hair-like strands of plastic. These xe2x80x9cstringersxe2x80x9d may then. break off at a later time and pass through the discharge conduit in a subsequent unloading operation. The presence of such non-uniformly sized xe2x80x9cstringersxe2x80x9d in the plastic material can create problems for the equipment that handles and processes the plastic and can contaminate a subsequent load of particulate material.
Various methods have been developed to deal with the problem of transport of temperature sensitive particulate materials. Thus, for example, it is known to employ a water-filled sheath around the conduit carrying the pressurizing air stream to the container. However, such a device requires that a supply of water be readily available at the unloading site, as well as the means for its disposal. It is also known to place cooling fins on the material conveying conduit. However, such cooling fins are bulky, heavy and expensive, and only marginally effective. It is also known to reduce the temperature created in a pressurizing system by restricting the operation of the blower or compressor to reduce the pressure produced. However, such restriction will slow the unloading process since the pressure differential between the container and the ambient conditions at the discharge end of the material conveying conduit will not be as great. Because of such reduced pressure in the container, the particulate material therein is also more likely to get clogged in the hopper outlets.
U.S. Pat. No. 5,580,193 and U.S. Pat. No. 5,779,398, both of Battle et al., and U.S. Pat. No. 5,618,136 of Smoot describe methods for reducing the temperature of the pressurizing air by placing a heat exchanger in the air stream. These methods require the employment of additional equipment, which increases the capital cost, operating expense and maintenance requirements for the system. The addition of heat exchanger components to a trailer-mounted container also increases the weight of the trailer, thereby reducing the payload it may carry as well as the fuel efficiency of the tractor-trailer combination. It would be desirable if a simpler method and apparatus could be provided that could be used to transfer particulate material from a container while minimizing any increase in the temperature to which the particulate material is exposed. It would also be desirable if such a method and apparatus could be provided that could be implemented by making only minor modifications of the conventional pneumatic container.
Among the advantages of the invention is that it provides a method and apparatus for the unloading of particulate material from a container while minimizing any increase in the temperature to which the material is exposed. Another advantage of the invention is that it provides such a method and apparatus which requires no significant addition of expensive equipment. Still another advantage of he invention is that it provides such a method and apparatus that does not add significant weight to a trailer-mounted container. Yet another advantage of the invention is that it provides such a method and apparatus utilizing only minor modifications to conventional pneumatic unloading systems.
Additional objects and advantages of this invention will become apparent from an examination of the drawings and the ensuing description.
As used herein, the term particulate material refers to granular, fluent or comminuted material that is capable of being transported through a conduit by an entraining gas.
As used herein, the term container refers to an enclosure for particulate materials that may include one or more product compartments.
As used herein the term pressurized gas refers to the air or gas that is introduced into the pressurized gas inlet of the container, directed to flow across at least a portion of the thermally-conductive shell of the container, withdrawn from the pressurized gas outlet and conducted to the material conveying conduit where it entrains material that has been discharged through the material outlet, regardless of the particular pressure conditions of such gas as it passes through such circuit.
The invention comprises a method and apparatus for unloading particulate material from a container having a thermally-conductive shell, an outlet for particulate material and a material conveying conduit connected to the material outlet. The apparatus includes a pressurized gas inlet and a pressurized gas outlet in the container, as well as a conduit between the pressurized gas outlet and the material conveying conduit. A source of pressurized gas or other means for introducing pressurized gas into the container is provided, so that pressurized gas may be introduced into the container through the pressurized gas inlet and directed to flow across at least a portion of the thermally-conductive shell. The pressurized gas is withdrawn from the pressurized gas outlet and conducted to the material conveying conduit. Particulate material is discharged through the material outlet into the material conveying conduit, where it is entrained by the pressurized gas and conveyed through the material conveying conduit. The container may be mounted on a frame of a trailer for transport.
In order to facilitate an understanding of the invention, the preferred embodiments of the invention are illustrated in the drawings, and a detailed description thereof follows. It is not intended, however, that the invention be limited to the particular embodiments described or to use in connection with the apparatus illustrated herein. Various modifications and alternative embodiments such as would ordinarily occur to one skilled in the art to which the invention relates are also contemplated and included within the scope of the invention described and claimed herein.